Replace Python numerical kernels with native C execution

bat/start-backend.bat

@@ -5,6 +5,7 @@ for %%I in ("%~dp0..") do set "REPO_ROOT=%%~fI"
 title SystemSimulationApp FastAPI - 127.0.0.1:8000
 
 set "PYTHON_EXE=%REPO_ROOT%\.venv-win\Scripts\python.exe"
+if not defined SIMULATION_NUMERIC_ENGINE set "SIMULATION_NUMERIC_ENGINE=native"
 
 if not exist "%PYTHON_EXE%" (
   echo [ERROR] Python virtual environment was not found: